1. Field of the Invention
The present invention relates to a control system for a servo circuit which performs speed control and then position control over an object under servo control (hereinafter) referred to as a "servo object"). More particularly, the present invention relates to a method for controlling the control speed of the servo object based on an error between a target speed and a real speed. The present invention also relates to a method for switching from speed control to position control, and a method for switching from speed control to position control when the real speed falls below a fixed value.
2. Description of the Related Art
Wide use is being made of servo circuits for the positioning of magnetic heads on tracks of magnetic disk devices and other precision positioning. In such servo circuits, there is a desire for a technique enabling stable positioning even at high movement speeds. Also, there is a demand for a method for stable switching control even in the face of external disturbances and other influences when switching from speed control to position control.
In recent years, magnetic disk apparatuses have tended to have higher speed positioning during seek (movement) operations. Due to these higher speeds, there is a need for increasing the degree of acceleration during deceleration. If the degree of acceleration during deceleration is increased, the entry speed at the final stage of the speed control will not be constant, but will fluctuate in accordance with the amount of the seek operation (movement). When the degree of acceleration during deceleration is small and a relatively long time is taken for the deceleration (for example, when over two times the acceleration time is taken during the deceleration), the entry speed substantially remains the same. But, if the degree of acceleration during deceleration is made large and the positioning time shortened, the entry speed fluctuates in accordance with the amount of the seek operations. These fluctuations in the entry speed create fluctuations in the amount of overshoot, invite fluctuations and prolongation of the convergence time in the fine control (position control), and end up halving the effects of the reduction of time.
Also, once the switching conditions are reached, the position control is switched unconditionally after a fixed time. Therefore, position control is switched even if the switching conditions only appear to have been reached due to disturbances in the position signal of a servo object due to external disturbances or if the position signal is disturbed by external disturbances after that. In such cases, overshooting or undershooting occur, position errors easily occur, too much time is taken for positioning, the reliability of the position control (seek) decreases, and the positioning (seek) time is increased.